As a rule, sensors or detectors for locating metallic objects concealed, for example, in building materials currently function with inductive methods. These methods make use of the fact that both conductive and ferromagnetic materials influence the properties of an electromagnetic coil brought into their vicinity. The changes in the inductive properties caused by metallic objects are registered by a reception circuit of such a detector. This makes it possible to basically locate metallic objects, for example, enclosed in a wall by moving one or more coils across the wall.
Magnetic and nonmagnetic metallic objects influence the inductive sensor in different ways. For example, the arrival of a ferromagnetic iron bar into the magnetic field of a sensor coil can be recognized by an increase in the inductance of the coil. By contrast, conductive, nonmagnetic materials, due to the eddy currents that are induced in them, cause an increase in the losses of the detector coils. The presence of conductive, nonmagnetic objects in the vicinity of the sensor can therefore be recognized by a reduction in the quality of the sensor coils. The impact that a metallic object has on the properties of an inductive sensor is generally a function of the frequency spectrum used for the sensor. Inductive sensors, which are primarily intended to react to ferromagnetic objects —in particular those made of iron and steel, function efficiently at frequencies in the range of 10 kHz and below since the magnetic susceptibility of most magnetic materials drops rapidly at higher frequencies.
One technical challenge in the detection of metallic objects is that the objects to be located have very little impact on the coil or coils of the sensor arrangement. This is primarily true for the influence of non-ferromagnetic objects such as the technically significant material copper.
As a rule, the detectors based on an inductive method have a high offset, i.e. a powerful signal can be picked up in the sensor, which is measured by the reception circuit of the detector even without the influence of an external metallic object, i.e. one to be located. Such a high offset makes it hard to detect very small inductive changes caused by a metallic object brought into the vicinity of the detector.
The change ΔL in the inductance L of a sensor coil that is caused by metallic objects is very slight in practice and in particular, is very much less than the inductance L that the same coil has in the absence of metallic objects. The greatest technical challenge in the detection, therefore, is not the low absolute value of the change in the sensor properties, i.e. the inductance change ΔL, but rather the often almost infinitesimally slight relative change in the sensor properties, i.e. an extremely small quotient ΔL/L.
The offset is a particular source of interference in the detection of nonmagnetic materials. The conventional way to achieve a high measuring capacity is to use particularly high quality sensor coils. The slight reduction in the coil quality caused by the object to be located is then only slightly concealed by the coil losses that are present anyway. The usual approach is comprised of using sensor reception coils with a large number of turns and ferrite cores with particularly low magnetic losses.
The need to detect a very slight change in the sensor properties in a very large offset signal requires the use of strictly toleranced and therefore expensive components for such a detector and also requires very low-drift analog electronics, both of which considerably increase the costs for the sensor and for the corresponding locating device.
In order to counteract this offset problem, numerous approaches have been taken in the prior art, all of which share the common goal of reducing the sensor signal that is present in the absence of metallic objects and thus increasing the relative signal changes.
Often, a multi-step approach is taken in which, for example in a first step, an arrangement of sensor coils is used, which has the capacity to completely eliminate or compensate for the signal offset in the ideal case. The compensation quality that can be achieved in practice, however, frequently depends on production tolerances so that a complete elimination of the signal offset frequently requires an additional process, for fine compensation so to speak.
The shared concept that forms the basis of the family of metal detectors collectively referred to below as “inductive compensation sensors” is comprised in constructing the detector out of more than one individual coil and in particular, distinguishing between a winding system for magnetic field excitation and a winding system for detection. In particular, it is possible to use arrangements that position inversely wound conductor loops in space so that the voltages induced in the reception system by the excitation magnetic field disappear in the absence of metallic objects. This is achieved, for example, by virtue of the fact that an excitation magnetic field induces voltages of the same amplitudes in inversely oriented reception conductor loops, but these voltages have opposite signs due to the orientation and therefore cancel each other out.
DE 101 22 741 A1 has disclosed a detector for locating metal objects, which has a reception coil and a first transmission or excitation coil that are inductively coupled to each other. In order to produce the smallest possible offset signal in the detector, a second transmission coil is provided, which is likewise inductively coupled to the reception coil. The reception coil and the two transmission or excitation coils are situated concentric to a shared axis; with regard to their numbers of turns, orientation, and/or dimensions, the two transmission coils are sized so that the fluxes they excite in the reception coil compensate for each other precisely in the absence of metallic objects.
U.S. Pat. No. 5,729,143 has disclosed a detector whose purpose is to suppress to the greatest extent possible the above-mentioned offset of the measurement signal. To this end, the detector in U.S. Pat. No. 5,729,143 has a transmission coil equipped with a transmitter and has a reception coil equipped with a receiver. The transmission coil and the reception coil of the detector are coupled to each other inductively so that they partially overlap each other. The transmitter supplies an alternating current to the transmission coil. Through its inductive coupling to the reception coil, this current-carrying transmission coil excites a first partial flux in the reception coil in the overlapping region of the two coils and excites a second partial flux in the remaining area of the reception coil. The distance between the centers of the transmission coil and reception coil can then be selected so that these two partial fluxes, which have opposite signs, precisely compensate for each other. If this is the case, then when there is no external metallic object in the vicinity of the coil arrangement, the current-carrying transmission coil does not induce any current in the reception coil so that in this ideal case, the receiver would also not measure any offset signal. Only if the coil arrangement is brought into the vicinity of a metallic object does the field generated by the transmission coil experience interference so that a non-negligible flux is then excited in the reception coil, thus generating a measurement signal in the reception coil, which measurement signal is not influenced by an offset signal and can be evaluated by the receiver or an evaluation circuit connected to it.
The relative spacing of the centers of the transmission coil and reception coil is thus an extremely critical parameter so that the ideally-to-be-assumed absence of an induced voltage in the reception coil can, in practice, only be implemented with a great deal of technical complexity. It has turned out that this approach does not permit implementation of a sufficient compensation of the flux components under the conditions of a series production of such a sensor.
For this reason, U.S. Pat. No. 5,729,143 proposes an electronic circuit that achieves the compensation afterwards by electronic means and also renders such a sensor practically usable.
The method described in U.S. Pat. No. 5,729,143 functions at a single frequency. On the excitation side, a magnetic alternating field of a particular frequency f is generated and the induced voltage components are evaluated in a frequency-selective fashion specifically at this frequency f in the reception windings by means of suitable analog and digital filters. At the frequency f, the voltage U(f) induced in the reception windings by the incorrect magnetic compensation of the reception and excitation system has a temperature-dependent amplitude and phase position that is also subject to additional series tolerances. The method in U.S. Pat. No. 5,729,143, then, is based on taking the voltages induced in the reception windings and adding a correction voltage to them in analog fashion, whose amplitude and phase position precisely compensate for the error voltage U(f) at the working frequency f. To this end, at the frequency f, a microprocessor generates a phase-controlled and amplitude-controlled correction signal. The amplitude and phase position required for the compensation in this case depend on the phase shift provoked by the components of the circuits in the excitation and reception branch. But as a result of this, the required correction signal is also subject to a temperature drift, among other things. In order to also be able to compensate for the error voltage U(D when there are changes in the working temperature, the microprocessor must track the phase position and amplitude of the correction signal over the temperature. As a rule, this requires recalibration of the sensor by the user.
EP 1092989 A1 has disclosed an alternative method for compensating for a residual offset that remains, for example, due to production tolerances. In lieu of adding a correction voltage to the detection voltage that is induced in the reception windings, this method works with additional corrective magnetic fields. To this end, the system of the magnetic field excitation is not comprised of just the primary excitation coil, but so-called trim windings and correction windings are also added. The difference between the trim winding and correction winding is that the correction windings are series connected to the primary excitation coil and therefore always have the same current flowing through them, whereas the so-called trim windings can be supplied with an adjustable fraction of the current flowing in the correction and excitation coils. In this way, it is possible for there to be no induced voltage in the detector coils with the absence of metallic objects in the vicinity of the sensor. The method in EP 1092989 A1 thus depends significantly less on component tolerances and drifts in the transmission and reception circuits. In addition, the measurement is not limited to a selected working frequency since the compensation is largely independent of the frequency used. But comparatively speaking, the design of a sensor according to EP 1092989 A1 is significantly more complex. While the sensor in U.S. Pat. No. 5,729,143 properly functions with only one coil each for the transmission and reception circuit, the design in EP 1092989 A1 requires ten coils in the transmission or excitation path and four coils for the reception path.
In compensation sensors, metallic objects initially generate signal changes of significantly smaller amplitude than in sensors that do not differentiate between an independent excitation and reception system. Not only do the voltages induced by the original excitation magnetic field cancel themselves out in the reception branch, but also the weak magnetic field changes clause by the object to be detected are generally subject to a certain degree of compensation.
Also in compensation sensors, strict limits are placed on the use of magnetic cores. The quality of the compensation in this case is highly dependent on the magnetic susceptibility of the ferrite used, which frequently cannot be toleranced strictly enough. In practice, therefore, air-cored coils are used as a rule in compensation sensors, where it is necessary to accept the fact that the coil quality decreases drastically and in particular, it is more difficult to detect nonmagnetic materials.
A standard approach for circumventing the problem of the weak signal and the low quality of air-cored coils in compensation sensors, as explained in detail in EP 1092989 A1, is to select a turns number that is as high as possible in reception coil systems.